Add flake examples for cross-compilation

[holly-hacker]

For the past hour I've been trying to convert the cross-aarch64 example to a flake, to no avail. It'd be nice if an example of this was provided.

[oxalica]

cross-aarch64 example to a flake

I don't think the current flake design supports cross-compilation without re-importing nixpkgs with a custom config, which also opt-out flake benefits like eval-caching. There are issues NixOS/nix#5157 and blogs about the current state of cross-compilation for flakes.

Alternatively to the re-importing with config/overlay solution, we can also provide a builder interface for rust-overlay to be used as (rust-overlay.lib.mkRustBin nixpkgs.legacyPackages.pkgsCross.aarch64-multiplatform).stable.latest. This will re-instantiate only rust-overlay packages rather than all nixpkgs packages. It still cannot use eval-cache and will instantiate every time for each flake input using rust-overlay, but at least it's the lesser of two evils.

The second solution is also mentioned in #94 (comment)

[holly-hacker]

Thanks for the fix!